In mobile communications, in the case of data communication in which retransmission control is carried out, it has been the conventional practice to adopt full duplex communication for a circuit switching mode which employs equal capacity channels which operate in opposite directions. FIG. 7 is a block diagram showing a structural example of a conventional mobile data communication system. This figure will now be used to explain the method by which terminal 11 of mobile station 10 transmits data to terminal 61 which is connected to a public telephone network. However, since terminal 11 and terminal 61 may be connected using a well-known procedure such as one determined according to RCR STD-27C, the digital automobile telephone system standard in Japan, for example, the following explanation will address a data transmission method where a fill duplex line is already established between terminal 11 and terminal 16.
First, terminal 11 of mobile station 10 supplies data (user data) to be transmitted to data communication unit 12. Data communication unit 12 forms a data frame which is used with radio transmission by adding an error detecting code and an error correction code to the user data which is supplied from terminal 11. Data communication unit 12 supplies a signal of the data frame to transmission and transmitter-receiver 13, and simultaneously records the user data for the retransmission. Transmission and transmitter-receiver 13 modulates the signal supplied from data communication unit 12, and transmits the modulated signal to base station 40. Transmission and transmitter-receiver 41 of base station 40 receives the modulated signal, demodulates the received signal, and then supplies the demodulated signal to control station of base station 50. Switching apparatus 51 of control station 50 relays a signal which is supplied from transmission and transmitter-receiver 41 to data communication unit 52.
Data communication unit 52 demodulates the user data, the error correction code and the error detecting code in the relayed signal (signal of aforementioned data frame). When data communication unit 52 is able to determine that there is no error in the user data, data communication unit 52 transforms the demodulated user data to a signal format for the public telephone network, and outputs the transformed data to terminal 61 through the public telephone network. On the other hand, when data communication unit 52 determines that the user data has an error, data communication unit 52 carries out retransmission between itself and data communication 12 of mobile station 10. In this way, then, retransmission control is carried out between data communication unit 12 of mobile station 10 and data communication unit 52 of control station 50.
Next, an explanation will be made of a conventional retransmission control method, FIG. 8 is a conceptual diagram showing the frame constitution of data (signal) which is transmitted and received between data communication unit 12 and data communication unit 52. An explanation will now be made of the operation of data communication unit 12 with reference to FIG. 8.
Data communication unit 12 first adds a forward control signal and a backward control signal to the user data upon receiving the user data from terminal 11. The forward control signal is a signal used in retransmission control which is carried out when data communication unit 52 of control station 50 does not correctly receive transmitted data from data communication unit 12. In general, the forward control signal consists of a frame number for the transmitted data and a retransmission signal recognition bit. The backward control signal is a signal for reporting to data communication unit 12 whether or not data communication unit 52 of control station 50 was able to receive the transmitted data correctly. In general, the backward control signal consists of a frame number for the received data or the data which failed to be received, and the receiving (ACK)/non-receiving (NAK) recognition bit.
Next, data communication unit 12 adds an error detecting code to the data unit, the data unit consisting of the user data signal, the forward control signal and the backward control signal. In other words, the data unit is encoded with an error detecting code. Data communication unit 12 forms a frame by encoding the data unit, which was encoded with an error detecting code, with an error correction code, and supplies the signal of the frame to transmission and transmitter-receiver 13.
FIG. 9 is a timing chart showing an example of the exchange of a transmission signal when data is transmitted and received using the frame construction. In FIG. 9, the transmitted frames from data communication unit 12 are sequentially designated frame A1, frame A2, . . . , while the transmitted frames from data communication unit 52 are sequentially designated frame B1, frame B2, . . . The signals to be transmitted in order from data communication unit 12 are designated forward signal a1, forward signal a2, . . . , and the signals to be transmitted sequentially at each frame from data communication unit 52 are designated forward signal b1, b2, . . . Each forward signal contains the user data signal and the forward control signal, and is transmitted after the backward control signal has been added thereto.
In FIG. 9, data communication unit 12 of mobile station 10 first transmits forward signal a1 at frame A1, next transmits forward signal a2 at frame A2, and then transmits; forward signal a3 at frame A3. Data communication unit 12 also receives forward signal b1 and backward control signal ACKa1 which are transmitted at frame B1 from data communication unit 52 of control station 50 in time with the transmission at frame A3, and confirms the details of the received signal. Herein, received backward control signal ACKa1 is the receiving acknowledgment signal of signal a1, and indicates that forward signal a1 was received correctly by data communication unit 52. When data communication unit 12 correctly receives forward signal b1, data communication unit 12 adds a receiving acknowledgment signal (backward control signal ANKb1) for forward signal b1 to forward signal a4 at frame A4, and transmits this signal. At this time, data communication unit 12 receives forward signal b2 and the backward control signal which were transmitted at frame. B2 from data communication unit 52 of control station 50.
Backward control signal NAKa2 is a retransmission request signal for forward signal a2, and indicates that forward signal a2 was not received correctly. Therefore, in the next transmission timing (frame A5), forward signal a2 is transmitted again. However, forward signal b2 was received correctly, so that backward control signal ACKb2, which is the receiving acknowledgment signal of forward signal b2, is added to forward signal a2 of the retransmission procedure.
The above-mentioned operation is carried out in the same way simultaneously in data communication unit 52 of control station 50, with two-way data communication carried out using the same channel (line).
As described above, it has been the conventional practice to adopt a full duplex communication for the circuit switching mode in the case of data communications employing a retransmission control method in mobile communications, with the backward control signal for controlling retransmission transmitted on the same channel as the forward signal (forward control signal and user data signal). However, it is generally rare that the traffic in data communications occurs at the same time in two directions. Rather, one-way data transmission is more frequent. Therefore, in the conventional method, a dummy signal is frequently transmitted in one direction of the full duplex line, giving rise to the problem that the utilization efficiency of the radio line is low.
As a method for resolving this problem, adoption of one-way communication in the packet switching mode may be considered. One popular method of one-way communication in the packet switching mode is a retransmission control method which, carries out retransmission of every packet or every message in the upper layer. However, this method has disadvantages in that the retransmission unit is large, and the transmission efficiency of the radio line is low. For example, a wireless LAN (Local Area Network) is able to realize packet data communication in the interval for radio, but adopts a retransmission control method for packet units. As a result, this method is applied under conditions of low transmission efficiency (see, for example, K. Pahlavan, "Trends in Local Wireless Data Networks", IEEE Vehicular Technology Conference 1996).